Heard two heartbeats using heart doppler at home??

At my last appt. I had a sonogram at 18 weeks, and I'm only having one baby. Earlier today, my husband and I were listening to our baby's heartbeat, and the first sound we picked up sounded like a heartbeat it was on my right side, it sounded to deep or low sounding to be the heartbeat, but we listened to our baby's heartbeat on my left side. What could that other sound have been? I thought it may have been mine, but it was too fast.